1. Field of the Invention
The present invention relates to a two-terminal surge protector suitable for protecting electronic circuits of various communication equipments from surge currents.
2. Description of the Related Art
In order to protect circuits of various electronic equipments from surge currents, a thyristor type two-terminal surge protector having a small size and a high speed operation has been used. In the thyristor type two-terminal surge protector, a thyristor having four layers of P.sub.1 N.sub.2 P.sub.3 N.sub.4 shown in FIG. 7A or a thyristor having a P.sub.1 layer and an N.sub.2 layer partially shorted as shown in FIG. 7B has been known. These thyristors have voltage(v)-current(I) characteristics (a) and (b) having a breakdown voltage V.sub.BO as shown in FIG. 7C. However, although a holding current I.sub.H2 of the thyristor having the short-circuited structure is larger than a holding current I.sub.H3 of the thyristor having no short-circuited structure, the breakdown voltage V.sub.BO of each of the protectors is lower than a breakdown voltage V.sub.B of a junction J.sub.2.
In order to protect an electronic circuit from a surge current by using the above thyristor type surge protector, as shown in FIG. 8A, a surge protector Z is connected between lines L.sub.1 and L.sub.2 in parallel with an electronic circuit G. In addition, in order to protect the electronic circuit from a bidirectional surge current S, as shown in FIG. 8B, the surge protector Z is connected between the lines L.sub.1 and L.sub.2 to which an electronic circuit G is connected through a diode bridge circuit DB. For protecting the electronic circuit G, having N Lines L.sub.1 to L.sub.N, from negative and positive bidirectional surge currents, one surge protector Z is commonly connected to the lines L.sub.1 to L.sub.N each through diodes D.sub.1 and D.sub.2, respectively, as shown in FIG. 8C.
When the surge voltage S applied to the lines exceeds the breakdown voltage V.sub.BO, the surge protector Z is turned on to cause a current to flow through the protector Z. The protector Z protects the electronic circuit G such that a voltage higher than the breakdown voltage V.sub.BO is not applied to the electronic circuit G. In fact, when a rise time dv/dt of the surge voltage S is high, as will be described later, the protector Z is turned on at an operating voltage (clamping voltage) V.sub.CL higher than the breakdown voltage V.sub.BO. The surge current flows through the surge protector Z to decrease the current flowing through the electronic circuit. When the current drops to the holding current I.sub.H, the protector Z returns to its normal noncondictive state, the current given by a line impedance R and a bias voltage E flows through the electronic circuit. The protector Z is thus prepared for a next surge current.
For carrying out more preferable protection by using the above two-terminal surge protector, the following conditions must be satisfied.
(1) In order to improve a breaking performance, the holding current I.sub.H is increased such that a relationship between the holding current I.sub.H and a current E/R given by the circuit impedance R and the bias voltage E satisfies a condition of I.sub.H&gt; E/R.
(2) A withstand surge current is increased.
(3) The capacitance of the surge protector connected between the lines is reduced to decrease the capacitance to be connected therebetween, thereby preventing degradation of communication performance caused by attenuation of a transmission signal.
(4) The surge operating voltage V.sub.CL, which is caused by increasing the rise time of the surge voltage and is increased by a breakdown voltage (DC blocking voltage) V.sub.BO, is decreased, thereby reducing the surge voltage applied to the electronic circuit G.
That is, in the structure of FIG. 7A, when the current amplification factors of transistors N.sub.4 P.sub.3 N.sub.2 and P.sub.1 N.sub.2 P.sub.3 are represented by .alpha..sub.1 and .alpha..sub.2, and a breakdown voltage of a junction between N.sub.2 and P.sub.3 is represented by V.sub.B, the breakdown voltage V.sub.BO is given by: EQU V.sub.BO .congruent.V.sub.B {1-(.alpha..sub.1 +.alpha..sub.2)}.sup.1/n
(where n.congruent.2 to 6)
In the structure of FIG. 7B, the breakdown voltage V.sub.BO is approximately given by: EQU V.sub.BO .congruent.V.sub.B (1-.alpha..sub.1).sup.1/n
For this reason, in either structures, as indicated by the characteristics (a) and (b) in FIG. 7C, V.sub.BO &lt;V.sub.B is obtained, so that the voltage V.sub.BO is lower than the voltage V.sub.B.
When the rise time of the normal surge voltage, i.e., the rise time dv/dt becomes about 100/.mu.s, the surge operating voltage V.sub.CL becomes equal to the voltage V.sub.B. Therefore, EQU V.sub.CL (=V.sub.B)&gt;V.sub.BO
is obtained.
In order to obtain more preferable surge protection performance, V.sub.CL =V.sub.BO is desirably satisfied while a decrease in voltage V.sub.BO is prevented as much as possible.
When each of the thyristor type surge protectors shown in FIG. 7A and 7B is turned on to allow the surge current of about 100 A to flow therethrough, both of them are not affected by the short-circuited structure, i.e., the withstand surge is not changed. However, when the current is decreased to about 100 mA to turn off such protectors after the surge current passes through, they are affected by the short-circuited structure. Therefore, the protector having the short-circuited structure is turned off by a current larger than that of the surge protector which has no short-circuited structure. That is, as shown by the characteristics (a) and (b), the turn-off current, i.e., the holding current I.sub.H is increased from I.sub.H3 to I.sub.H2 in accordance with the presence/absence of the short-circuited structure.
In addition, the above conditions have a strict trade-off relationship in design of the protector. That is, by defining the impurity concentration, thickness, and the like of the P.sub.1 N.sub.2 P.sub.3 N.sub.4 layers shown in FIG. 7A, the amplification factors .alpha..sub.1 and .alpha..sub.2 of the transistors can be reduced to suppress a decrease in the breakdown voltage V.sub.BO from the voltage V.sub.B. In this case, however, the withstand surge current is decreased.
When the capacitance of the surge protector is decreased to prevent the degradation of communication performance, the voltage V.sub.BO is increased. In the conventional structures, it is difficult to eliminate the trade-off relationship and to satisfy all of the conditions described above.